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diy solar

2 strings maximum?

dmpiowa

New Member
Joined
Mar 23, 2022
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Hi,

I am new to solar and I was looking at buying some used panels to add to my system. I have a Growatt SPF 5000 US. It will handle 6000 watts at 450 volts Maximum with a max of 100 amps on the charging circuit. I am looking at hooking up 27 - 225 watt used panels in three strings of 9 panels for a total wattage of 2025 watts per string. The system would produce a total of 6075 watts at 331.92 volts with an open circuit amperage of 24.81 amps. I was going to combine these in a combiner box and run the PV back through 10awg wires in conduit to the inverter with 15 amp fuses on each positive wire on the string. When I went to order the panels I was told that the Growatt can only handle two strings of panels and if I add the third string that I won't see any gain from from it.

Panel specs are as follows: output = 225 watts, open circuit voltage (VOC) = 36.88v, Short circuit current (ISC) = 8.27a

Is this true? This doesn't seem right to me, but I am just a newbie so I am asking the experts here. If this IS true I may switch up horses to another used panel that is 250 watts and look at making two strings of 11 panels.

Panel specs Rated Power: 250W, Open circuit voltage (VOC): 37.6 V, Short circuit current (ISC): 8.85 A,

Two strings of 12 would be 6000 watts, 17.7 amps but 451.2 volts, Over my 450 volt maximum.
So I would be looking at 2 strings at 413.6 volts per string and 2750 watts per string with 8.85 ams per string.
Total would be 5,500 watts, 413 volts and 17.7 amps. That puts my amps pretty close to max on a very cold Missouri day.....

So can I do three strings, or do I need to focus on two? Thanks, Daniel
 
Last edited:
So I would be looking at 2 strings at 413.6 volts per string
Make sure you take into consideration that as the temp drops below 25C (77F) the panels will produce more voltage. Exceeding the SCC max input voltage is normally very bad.

I wish i understood more about the issue with 3 strings into your SCC. Are there 2 distinct MPPTs in the Growatt?

Conjuring @FilterGuy who has this unit
 
Make sure you take into consideration that as the temp drops below 25C (77F) the panels will produce more voltage. Exceeding the SCC max input voltage is normally very bad.
332 V on a 450V PV circuit leaves over 30% for cold temp voltage gain. That will be fine. I would have no worries about voltage gain.

However, 451V would be a problem in warm weather and a disaster in cold weather.

As far as 2 strings vs 3 strings.... I can't imagine how the inverter would even know how many strings there are, let alone care. As long as the array does not exceed the total voltage and power of the inverter spec it will be fine. In your case the 250W/panel x 27panels = 6075W. This is slightly greater than the spec, but it will probably work just fine. (Many people over-panel the Growatts even though Growatt says you can't)
 
There is only 1 PV input on the Growatt SPF 5000 US. I couldn't see how the PV Controller would be able to tell the difference between 2 strings or three after it comes out of the combiner box, but I am a newbie and thought maybe I was missing something? Perhaps they are just wanting to sell me higher watt panels. Thanks!
 
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